I agree with Gotural. The difference relates to whether you're considering the probability of: (i) the RNG rolling all 18s or (ii) that the result displayed will be all 18s.
The former is simple to calculate as suggested above. However, as we're talking about playing the game, the second is more relevant here. That probability is much more complex to calculate due to the interaction of the various class/race minima and the overall minimum of 75 and will vary considerably between different characters. However, the chance of seeing all 18s displayed will in all cases be much higher than 0.00000000000000984640 - not that the difference is terribly relevant when such small numbers are being considered .
It might be helpful to illustrate the difference between (i) and (ii) further by thinking of creating a new class, which has minimum stat requirements of 18 across the board. When you ask the game to make a 'random' roll for a new character with this class the chance of it producing all 18s with its first displayed roll will be 100% - as any roll that does not meet the class requirements is discarded before being displayed. It might actually be interesting to adjust class requirements like that to see how long it would take the game to roll a character - that might provide insight to whether the game does simply roll all 6 stats at once and discard the whole roll if it doesn't meet requirements, or whether there is some intermediate processing stage as some have suggested.
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(i) the RNG rolling all 18s or
(ii) that the result displayed will be all 18s.
The former is simple to calculate as suggested above. However, as we're talking about playing the game, the second is more relevant here. That probability is much more complex to calculate due to the interaction of the various class/race minima and the overall minimum of 75 and will vary considerably between different characters. However, the chance of seeing all 18s displayed will in all cases be much higher than 0.00000000000000984640 - not that the difference is terribly relevant when such small numbers are being considered
It might be helpful to illustrate the difference between (i) and (ii) further by thinking of creating a new class, which has minimum stat requirements of 18 across the board. When you ask the game to make a 'random' roll for a new character with this class the chance of it producing all 18s with its first displayed roll will be 100% - as any roll that does not meet the class requirements is discarded before being displayed. It might actually be interesting to adjust class requirements like that to see how long it would take the game to roll a character - that might provide insight to whether the game does simply roll all 6 stats at once and discard the whole roll if it doesn't meet requirements, or whether there is some intermediate processing stage as some have suggested.